For the past few decades, the development of efficient luminescent materials, having desirable optoelectronic properties, has been a topic of great interest. The development of efficient luminescent materials, however, has been hindered by problems associated with aggregation-caused quenching of light emission which is notorious for rendering luminophors ineffective for solid state applications, particularly those involving electroluminescent (EL) devices. In order for luminescent materials to have practical applications as electron-transporting or emissive layers as thin films in semiconducting and electronic devices, a luminophor should exhibit high fluorescence quantum yield (ΦF) in the solid state. Many organic fluorophores experience aggregation-caused quenching (ACQ) of light emission in solution as well as the solid state as a result of interactions with neighboring fluorophores which promote the formation of delocalized excitons or excimers which decay non-radiatively. As a result, low concentrations of the fluorophore molecule must be used in order to minimize contact between adjacent molecules to mitigate the ACQ effect resulting in decreased sensitivity and reliability of the fluorescent signal. A logical approach to alleviating the problems associated with ACQ would be to develop luminophors whose aggregates fluoresce more strongly than their solutions. Identification of two photoluminescence processes, aggregation-induced emission (AIE) and aggregation-induced emission enhancement (AIEE), may now allow development of highly efficient solid state fluorescence. In AIE, a non-emissive chromophore is induced to emit light by the formation of aggregates while the light emission of an AIEE molecule is significantly enhanced once aggregation occurs.
Silacyclopentadienes, or siloles (FIG. 1, M=Si), are a class of molecules that have been previously extensively developed for their potential application in organic electronics, particularly in flexible lighting and display panels. One of the qualities responsible for the intense interest in siloles is the high electron affinity that these cyclic molecules exhibit. Such large electron affinities can be attributed to a low lying LUMO which arises from σ*-π* conjugation that results from the interaction between the π* orbital of the butadiene segment and the σ* orbital associated with the two exocyclic bonds on the silicon center. The large electron affinity of the siloles results in another favorable feature, high electron mobility, a desirable attribute that continues to present challenges in the design of highly efficient organic electronic devices. There are many silole derivatives that have been reported to be good electron transporters with electron mobilities that are two orders of magnitude higher than tris(8-hydroxyquinolinato)aluminum (Alq3). Alq3 is a commonly used electron-transport (ET) material for organic light-emitting diodes (OLEDs).
Siloles have demonstrated high photoluminescence (PL) quantum yields as both amorphous and crystalline solids which can be attributed to the unique photophysical property of aggregation-induced emission (AIE). As a result of the steric repulsions between the peripheral aryl substituents on the core ring, intramolecular rotations of the substituents are restricted causing the substituents on the silole core to assume a highly twisted conformation that persists in solution as well as the solid state. Restriction of intramolecular rotations of the peripheral substituents effectively blocks non-radiative relaxation channels and imparts non-planarity, rendering the distance between adjacent silole molecules too long for conventional π-π stacking interactions (˜3-4 Å) that typically quench luminogens in the solid and crystalline phases. This mechanism is referred to as restricted intramolecular rotation (RIR) and is the accepted cause of the AIE phenomenon. RIR so effectively deactivates the avenues that result in non-radiative emission that siloles strongly emit light in the solid and crystalline phases, such as aggregated suspensions in solvent-water mixtures.
The AIE effect has now been identified in other luminogens with similar structural features including germoles, the heavier Group 14 congener of siloles (FIG. 1, M=Ge). Although germoles emit more efficiently in solution than siloles, their solutions are still only weakly emissive. The increased efficiency in solution, however, does not diminish the significant AIE effect that is exhibited by germoles in the solid state or when aggregated in solvent-water mixtures. Germoles, like siloles, are soluble in a variety of common organic solvents, but insoluble in water.
To date, relatively little has been published on the PL of germoles, despite published evidence of the similarities between siloles and germoles. In addition to exhibiting the AIE effect, parallels between the electronic structure and photophysical properties of the two metalloles can be seen by the similarities in the UV-vis absorption and fluorescence profiles, electrochemical data, and ab initio calculations of HOMO and LUMO energy levels. Such studies indicate comparable σ* (Si—R) and σ* (Ge—R) orbitals as well low lying LUMO energy levels, suggesting that the differences in the electronic structures of germanium and silicon analogs are relatively minimal despite the slightly larger size of the germanium atom. This is in contrast to the attributes that would be gleaned from the low lying LUMO in stannoles which are diminished by significantly less efficient σ*-π* conjugation that results from a greater orbital mismatch as well as elongated bond distances between the larger 5pz σ* orbital of tin and the 2pzπ* orbital of the carbons of the butadiene.
Therefore, there is a need to develop a series of compounds containing a germanium ring core, and in particular, germoles, exhibiting intense fluoresce quantum yields as aggregates in solution or in the solid state to be employed in light-emitting devices and luminescent sensors.